GaN nanowires have been the subject of intense research lately, due to the many potential ultraviolet applications and
interesting properties that they possess. Because GaN has an anisotropic wurtzite crystal structure, many of its properties
are dependent upon crystal orientation. For example, the photoluminescence (PL) of GaN nanowires with growth
direction along the a-axis is blue-shifted relative to the PL of wires with growth direction along the c-axis. However, the
origin of the difference in PL between nanowire samples of different growth directions remains unclear. To determine if
surface states play a role in the dependence of GaN nanowire photoluminescence on crystal orientation, we use time-integrated
photoluminescence (TIPL) and time-resolved photoluminescence (TRPL) to study the PL from GaN nanowire
samples of different crystallographic orientations. We observe temporal dynamics of the blue-shifted PL feature in the a-axis
GaN nanowires that is suggestive of a surface trapping process occurring, where some fraction of electron-hole
pairs are prevented from recombining via the band edge emission process because carriers diffuse to the surface where
they are trapped before carrier relaxation to the band edge is complete. Once a carrier is trapped and localized at a
surface trap state, light emission primarily occurs only when the complementary carrier diffuses to the same surface trap.
We envision that a thin oxide layer forming at the surface introduces surface traps that cause the blue emission, and that
the surfaces of the a-axis GaN nanowires are more susceptible to this oxidation than the c-axis GaN nanowire surfaces.
Here, we report a direct synthesis approach for obtaining GaN nanowires with control on growth directions: <0001> or
c-direction, and <10-10> or a-direction, on amorphous substrates. The direct nitridation of Ga droplets using either
dissociated ammonia or N<sub>2</sub>/H<sub>2</sub> plasma resulted in GaN nanowires with <0001> growth direction; and the vapor transport
of controlled (low) amounts of Ga flux in the presence of dissociated ammonia resulted in GaN nanowires with <10-10>
growth direction. In both cases, the resulting GaN nanowires have diameters as small as 20 nm and lengths exceeding
one hundred microns. Photoluminescence measurements showed that the bandgap of <10-10> wires blue-shifted by 50
meV from the wires with <0001> direction. Homo-epitaxial growth studies onto the pre-synthesized a-direction GaN
nanowires led to belt or ribbon shaped morphologies. Homo-epitaxial growth onto c-direction wires developed micro
hexagonal prism morphologies. The island growth morphologies observed on the hundred micron long, sub 30 nm size
nanowires suggest that the surface transport of adatoms on c-direction wires exhibit ballistic transport or "one-dimensional"
transport with mean distances over several tens of microns.